Effects of Fe doping on ac susceptibility of Pr_0.75Na_0.25MnO₃

Ac susceptibility at low temperatures of Pr_0.75Na_0.25Mn_1-xFe_xO_3 (0 \le x \le 0.30) is investigated. The peak value of the real component of ac susceptibility \chi ' at the freezing temperature T_\rm f is suppressed with the increasing frequency. The peak value of \chi ' shows a linear relation between T_\rm f and the logarithm of the frequency \omega. The normalized slope P = \Delta T_\rm f /T_\rm f\Delta \rm lg\omega , which is much lower than canonical insulating spin glass systems in which 0.06 \le P \le 0.08. The peak value of the imaginary component of the ac susceptibility \chi'' at T_\rm f for the x = 0, 0.02, 0.30 samples increases with increasing frequency, suggesting a cluster glass ground state with a coexistence of charge-ordered phase and correlated ferromagnetic clusters in spin glass matrix. The peak value of \chi'' at T_\rm f for the x = 0.10 sample decreases with increasing frequency, suggesting a phase separation ground state. The peak value of \chi'' at T_\rm f for the x = 0.05 sample decreases with increasing frequency for \omega \le 52 Hz and increases subsequently till 701~Hz, and then decreases with further increasing frequency for \omega \ge 1501 Hz. This complex behaviour is ascribed to the competition between the effects of large and little ferromagnetic clusters in the sample. The ground state of x = 0.05 sample is a transition state from cluster glass to phase separation.